The continual demand for enhanced integrated circuit performance has resulted in, among other things, a dramatic reduction of semiconductor device geometries, and continual efforts to optimize the performance of every substructure within any semiconductor device. A number of improvements and innovations in fabrication processes, material composition, and layout of the active circuit levels of a semiconductor device have resulted in very high-density circuit designs. Increasingly dense circuit design has not only improved a number of performance characteristics, it has also increased the importance of, and attention to, semiconductor material properties and behaviors.
Through use, the operation of a transistor may degrade over time. There are currently several known modes of transistor degradation. One type of degradation mechanism involves channel hot carriers (CHC) and hot carrier injection (HCI). In general, an excessively high electric field within a transistor causes degradation, usually in the gate oxide. One type of degradation mechanism is referred to as off-state band-to-band tunneling induced hot-carrier degradation. In this type of degradation the most significant degradation can occur when the transistor is in the off-state (i.e., when the gate voltage is below Vt and the transistor is not conducting current).
Commonly device manufacturers specify or define a number of boundary device design parameters (e.g., max/min voltage, max/min current) within which a desired device reliability level may be achieved, or even guaranteed. For example, a semiconductor device may be guaranteed an operational life of 10 years if its supply voltage is maintained at or below 5 Volts over that life. Often, such specifications are derived from a number of characterization tests and simulations performed on sample devices or device structures.
In addition, many end equipment applications demand a guaranteed operational lifetime for a device operating at some fixed set or range of operating conditions. Where a semiconductor manufacturer is supplying devices utilizing a mature fabrication technology, a certain amount of historical data on the actual performance or degradation of the devices over some given lifetime may be available. Frequently, however, the manufacturer is producing the devices utilizing a new, state-of-the-art fabrication technology. In many cases, such technologies have not been in existence long enough to have actual lifetime performance or degradation data compiled. The device manufacturer must, nonetheless, determine some operational device lifetime that it will guarantee.
Manufacturers thus commonly rely on accelerated stress testing of sample device structures or devices. Such structures are dynamically stressed to levels far above their intended operating conditions, and data on critical operational or behavioral parameters at those dynamic stress levels is compiled. That data can then be evaluated to develop characterizations or profiles of the device technology, from which the manufacturer may extrapolate to provide some guaranteed lifetime at normal operating conditions.
Unfortunately however, the ability of a manufacturer to accurately characterize certain device operational or behavioral parameters independently has been somewhat limited by conventional characterization methodologies. Depending upon the manufacturing technology and upon the particular device structures being characterized, conventional characterization schemes may limit a manufacturer's ability to vary certain parameters independently during stress testing. As a result, characterizations of two or more parameters are often interdependent. Certain assumptions must then be made regarding the behavior of those parameters with respect to one another in order to evaluate and extrapolate characterization data. In a number of cases, those assumptions introduce a certain margin of error into characterization data. This margin of error can result in, for example, an overestimation or underestimation of the operational lifetime of a production device. Either situation is undesirable, subjecting either the device or end equipment manufacturer to unnecessary system failures or yield losses.
As a result, there is a need for a dynamic stress characterization system that effectively and accurately assesses degradation parameters independently—decoupling variances in operational or behavioral parametric values from one another and providing optimal device characterization in an easy, efficient and cost-effective manner.